1. Field of the Invention
The present invention relates to a photomask used in a photolithography process or the like in an LSI manufacturing process.
2. Description of the Background Art
FIG. 1 is a sectional view showing a structure of this type of conventional photomask. In FIG. 1, reference numeral 1 denotes a transparent substrate like a quartz substrate. Light blocking patterns 2 like a chrome pattern defining, for example, a wire pattern and phase shifters 3 for shifting the phase of a transmitted light by 1/2 wavelength are formed on the transparent substrate 1. The phase shifters 3 are placed in every other light transmitting portion 4 between the adjacent light blocking patterns 2.
In the operation of this embodiment, when light 5 is incident upon the transparent substrate 1, the light blocking pattern 2 blocks a part of the incident light 5. Further, the incident light 5 is partly inverted in phase by the phase shifters 3 and passes through it as outgoing light 6.
FIG. 2(a) shows an amplitude of an optical image of the outgoing light 6 just after outgoing from the transparent substrate 1 and the phase shifters 3. The outgoing light 6 is directed through an optical system 7 to a semiconductor wafer (not shown) which is coated with photoresist. Thus, the photoresist is exposed to light according to the pattern of the light blocking patterns 2. FIG. 2(b) shows an amplitude of an optical image of the outgoing light 6 after passing through the optical system 7. Lights refracted from the light transmitting portions 4 on opposite sides of a light blocking portion 8 to the light blocking portion 8 where light is blocked by the light blocking pattern 2 cancel each other, because they are opposite in phase. As a result, the resolution and the depth of focus are improved.
In the conventional photomask having a structure as stated above, a grouped pattern would be effective in improving the resolution and the depth of focus. However, there is the problem that an isolated pattern could not attain such an effect. As shown in an encircled part A in FIG. 2(b), since an edge of an optical image is not sufficiently steep, in other words, the full width at half maximum of the optical image is relatively large, there arises the problem that the boundary of photoresist photosensing/non-photosensing is easily varied, and the obtained resist pattern is insufficiently stable.